The present invention relates to a zoom lens having a high magnification, a compact size, low cost and high performance used suitably for a video camera, and to a method for manufacturing the same.
Conventionally, as a zoom lens used for a video camera, a zoom lens composed of four groups of lenses is known, as can be seen in JP2(1990)-55308A. The four groups include: a first lens group having a positive refracting force that is fixed; a second lens group having a negative refracting force that is movable for varying magnification; a third lens group having a positive refracting force that is fixed; and a fourth lens group having a positive refracting force that is movable for correcting fluctuations of an image plane resulting from varying magnification and also for focusing. The first, second, third and fourth lens groups are disposed from an object side in this order.
In addition, JP4(1992)-104114A (hereinafter referred to as a conventional example 1) discloses a zoom lens composed of four groups of lenses as described above in which one glass aspherical lens is used for the third lens group and the fourth lens group respectively mainly for the purpose of miniaturization. However, since a glass aspherical lens needs to be formed by a molding method at a high temperature, an expensive metal mold has a short lifetime, and the cost for a glass aspherical lens is about three times as high as for a glass spherical lens.
Furthermore, JP9(1997)-311272A (hereinafter referred to as a conventional example 2) uses a plastic lens for cost reduction. However, a plastic lens has a large coefficient of linear expansion and is also susceptible to a temperature change, so that its performance with respect to an image pickup device with a large number of pixels is not sufficient. Moreover, due to its low refractive index, a plastic lens is not suitable for miniaturization.
Furthermore, JP11(1999)-194273A (hereinafter referred to as a conventional example 3) proposes a method for manufacturing a zoom lens at low cost by replacing the glass aspherical lens in the third lens group of the conventional example 1 with a plastic aspherical lens. However, the problem was that this zoom lens is susceptible to deformation caused by a temperature change as in the conventional example 2.
As described above, a glass aspherical lens used for miniaturizing a zoom lens for a video camera was disadvantageously expensive. Moreover, when the expensive glass aspherical lens was replaced with an inexpensive plastic lens, due to the fact that plastics have a coefficient of linear expansion of about ten times as large as that of glass, the problem was that the effects of fluctuations in the refractive index or deformation caused by temperature changes were serious.
It is an object of the present invention to provide a zoom lens having a compact size while maintaining a high magnification ratio of about 20 times and a brightness of about F1.6, which at the same time is less susceptible to a temperature change, capable of realizing high performance at low cost and highly competitive from a practical standpoint.
To achieve the above-mentioned object, the present invention has the following structure.
A zoom lens of the present invention includes a first lens group having a positive refracting force that is fixed; a second lens group having a negative refracting force that is movable along an optical axis and provided with a function of varying magnification; a third lens group having a positive refracting force that is fixed; and a fourth lens group having a positive refracting force that is movable along an optical axis and provided with a function of correcting fluctuations of an image plane resulting from varying magnification or changing an object distance, the first, second, third and fourth lens groups being disposed from an object side in this order, wherein the third lens group and the fourth lens group both include a positive glass spherical lens and a plastic aspherical lens.
According to this configuration, when the refracting power of each lens group is enhanced for miniaturization, the refracting power needed therefor can be provided to the side of the glass spherical lens. Moreover, the plastic lenses provide aspherical surfaces needed for correcting aberration that increases particularly in the latter groups (the third group, the fourth group). Thus, by combining the glass spherical lens with the plastic aspherical lens, the effects of fluctuations in the refractive index or deformation caused by temperature changes which were regarded as disadvantageous in the conventional plastic lenses can be reduced, and by taking advantage of the properties of an inexpensive plastic lens, a compact zoom lens of high performance with a high magnification ratio of 20 times can be realized at low cost.
In the above-mentioned zoom lens, it is preferable that the plastic aspherical lens in the third lens group and the plastic aspherical lens in the fourth lens group satisfy the following conditional expressions:
fw/|f3p| less than 0.02
fw/|f4p| less than 0.02
where fw is a focal length of an entire system at a wide-angle end, f3p is a focal length of the plastic aspherical lens in the third group, and f4p is a focal length of the plastic aspherical lens in the fourth group.
By weakening the refracting power of the plastic aspherical lens used for the third lens group as well as for the fourth lens group in such a manner, the effects of fluctuations in the refractive index or deformation caused by temperature changes can be reduced, so that a zoom lens that is less susceptible to the use environment and has a compact size and high performance can be realized at low cost.
Furthermore, it is preferable in the above-mentioned zoom lens that the third lens group includes 2 lenses in 2 groups composed of a positive glass spherical lens and a plastic aspherical lens disposed from an object side in this order. By disposing a plastic aspherical lens in a position where axial marginal rays are almost afocal in the third group in such a manner, the effects of fluctuations in the refractive index or deformation caused by temperature changes can be reduced greatly, so that a zoom lens that has a compact size and high performance can be provided at low cost.
Next, a method for manufacturing a zoom lens of the present invention is characterized by including a third lens group and a fourth lens group respectively having a positive glass spherical lens and a plastic aspherical lens while using at least one or more lenses or lens barrels of an arbitrary zoom lens that is constructed of a first lens group having a positive refracting force that is fixed, a second lens group having a negative refracting force that is movable along an optical axis and provided with a function of varying magnification, a third lens group having a positive refracting force that is fixed, and a fourth lens group having a positive refracting force that is movable along an optical axis and provided with a function of correcting fluctuations of an image plane resulting from varying magnification or changing an object distance, the first, second, third and fourth lens groups being disposed from an object side in this order. Here, as the arbitrary zoom lens using lenses or lens barrels, for example, the zoom lenses shown in the above-mentioned conventional technologies can be used. As examples, there are a zoom lens in which the first lens group and the second lens group are composed entirely of glass spherical lenses, a zoom lens in which positive glass aspherical lenses are disposed for the third lens group and the fourth lens group, or a zoom lens in which plastic lenses with comparatively large refracting power are provided for the third lens group and the fourth lens group. In particular, the zoom lens preferably is a zoom lens in which positive glass aspherical lenses are provided for the third lens group and the fourth lens group. In addition, the first lens group and the second lens group preferably are composed entirely of glass spherical lenses.
According to the manufacturing method of the present invention, when both the zoom lens of the present invention mentioned above and a zoom lens other than this type (for example, a conventional zoom lens composed of four groups using positive glass aspherical lenses for the third group and the fourth group) are to be manufactured, components (lenses or lens barrels) other than the third lens group and the fourth lens group can be shared partially or entirely, so that a zoom lens having a compact size and high performance can be manufactured at low cost using fewer metal molds or with less capital investment. For example, since it becomes possible to provide a zoom lens that is suited for a high number of pixels in an image pickup device at low cost, it is particularly effective for conforming to merchandise lines of a video camera.
In the above-mentioned manufacturing method, it is preferable that the plastic aspherical lens in the third lens group and the plastic aspherical lens in the fourth lens group satisfy the following conditional expressions:
fw/|f3p| less than 0.02
fw/|f4p| less than 0.02
where fw is a focal length of an entire system at a wide-angle end, f3p is a focal length of the plastic aspherical lens in the third group, and f4p is a focal length of the plastic aspherical lens in the fourth group.
By weakening the refracting power of the plastic aspherical lens used for the third lens group as well as for the fourth lens group, the effects of fluctuations in the refractive index or deformation caused by temperature changes can be reduced, so that a zoom lens that is less susceptible to the use environment and has a compact size and high performance can be obtained at low cost.